Refractory metal silicides have been used in microelectronic applications by reacting refractory metals with silicon. The more popularly used refractory metal silicides include tungsten silicide (WSi.sub.2), titanium silicide (TiSi.sub.2), cobalt silicide (CoSi.sub.2), molybdenum silcide (MoSi.sub.2) and tantalum silicide (TaSi.sub.2). Based on their low resistivities and stability when in contact with polysilicon gates and junctions, WSi.sub.2, TiSi.sub.2 and CoSi.sub.2 are most commonly used silicides. To form a silicide selectively over exposed silicon regions, a refractory metal such as W or Ti can be first deposited over the entire wafer by a technique of sputtering, evaporation, or chemical vapor deposition (CVD). The deposition process is then followed by a two-step furnace or rapid-thermal annealing. For titanium, for example, the first annealing step consists of annealing at a temperature of 650.degree. C. for about 20 minutes in a nitrogen atmosphere while nitrogen reacts with titanium to form titanium nitride at the surface of the metal. Simultaneously, titanium reacts with silicon and forms silicide in the regions where titanium is in contact with silicon.
WSi.sub.2 is used widely on top of gate polysilicon to form a low-resistance polycide gate. The word polycide is used to indicate the combination of a layer of refractory metal silicide deposited on a layer of polysilicon. WSi.sub.2 are usually deposited by a sputtering technique or a CVD technique for use in ULSI devices. For the CVD method, WSi.sub.2 can be readily formed by using a silane reduction of tungsten fluoride (WF.sub.6) at a relatively low temperature, i.e., between about 300.degree. C. and 400.degree. C. As in most CVD reactions, the flow rates of WF.sub.6 and SiH.sub.4 control the outcome of the reaction. A higher SiH.sub.4 /WF.sub.6 ratio usually results in WSi.sub.2 deposition. In practice, a high ratio of SiH.sub.4 /WF.sub.6, i.e., greater than 10, is used to ensure the deposition of WSi.sub.2. CVD WSi.sub.2 is widely used on gate polysilicon and must survive high temperature exposures, i.e., 800.degree. C. to 1000.degree. C. during source/drain formation and reoxidation. WSi.sub.2 films produced from a silane reduction process has relatively poor step coverage. In addition, the high fluorine and chlorine content in the film tend to cause diffusion problems with the gate oxide and band shifts.
To compensate for these problems, WSi.sub.2 can be deposited by a reduction reaction of dichlorosilane (SiH.sub.2 CL.sub.2, or DCS) at a higher reaction temperature, i.e., between about 500.degree. C. and about 600.degree. C. WSi.sub.2 films deposited by the dichlorosilane reduction process contains less fluorine and chlorine than those from silane reduction process. While the resistivity and the film stress are comparable to those formed from silane reduction, WSi.sub.2 deposited from DCS reduction has a better step coverage. As a result, many of the disadvantages of WSi.sub.2 films formed by the silane process can be avoided and better deposition quality and process control can be achieved. The WSi.sub.2 films produced from the DCS process therefore gradually replacing those produced from the silane process.
A typical WSi.sub.2 polycide gate formation process is shown in FIGS. 1.about.3. In FIG. 1, a semiconductor device 10 is shown which consists of a silicon substrate 12, field oxide dielectric layers 14, and a gate oxide layer 16 deposited on top of the silicon substrate 12. In a typical WSi.sub.2 CVD deposition process, as shown in FIG. 2, a polysilicon layer 22 is deposited on top of the gate oxide layer 16. The polysilicon layer 22 may be doped in-situ to further improve its conductivity. On top of the polysilicon layer 22, a refractory metal silicide layer 24 is then deposited by either a sputtering or a chemical vapor deposition technique. The most commonly used polycides are WSi.sub.2, TaSi.sub.2, and MoSi.sub.2. These polycides are refractory, thermally stable, and resistant to most processing chemicals. In the next fabrication step, the polysilicon layer 22 and the refractory metal silicide layer 24 are patterned and formed into a gate structure 30. This is shown in FIG. 3.
After a polycide gate is formed, the gate structure must be isolated by depositing a dielectric layer on top. A most frequently used dielectric layer for such purpose is a silicon dioxide layer which can be deposited by a chemical vapor deposition process. While silane can be readily used to react with oxygen at relatively low reaction temperatures, i.e., below 500.degree. C., to form silicon dioxide, the composition of the film formed is not stoichiometric and as a result, the film exhibits a low dielectric breakdown field and furthermore it has poor step coverage capability. A more desirable process for forming a stoichiometric oxide film can be performed by reacting dichlorosilane with nitrous oxide by using a large excess of N.sub.2 O at lower pressures. However, the reaction temperature required for the dichlorosilane process is in the range between 700.degree. C..about.900.degree. C., which is in excess of the melting temperatures of typical metals used for interconnections such as aluminum and copper.
In either the low temperature formation of silicon dioxide films by the silane process, or the high temperature formation of silicon dioxide films by the DCS process, a problem is frequently encountered on the surface of the refractory metal silicide gate. The problem is especially pronounced when the silicon dioxide layer is deposited on a tungsten silicide gate formed by DCS, i.e., the transformation of the surface layer of the tungsten silicide gate from a smooth, dense material to a fibrous, sponge-like material. The fibrous, sponge-like surface layer of WSi.sub.2 is porous and therefore creates serious performance problems for the polycide gate. A fibrous, sponge-like surface layer of WSi.sub.2 is illustrated in FIG. 4. It should be noted that the Figure is used for illustrative purpose only, and as such is not drawn to scale. The fibrous, sponge-like surface texture 34 appears after the deposition step for the silicon dioxide sidewall spacers 42.
The formation of the fibrous, sponge-like surface texture in WSi.sub.2 has not been observed when the WSi.sub.2 film is formed from a silane based reaction. Since the major difference between the WSi.sub.2 film formed from a silane based reaction and that from a DCS based reaction is in the structure of the film, i.e., an amorphous structure is formed from the silane based reaction and a crystalline structure is formed from the DCS based reaction, the formation of the fibrous, sponge-like surface texture in WSi.sub.2 is presumably caused by the crystalline structure of the film. When depositing the oxide layer by a DCS/N.sub.2 0 reaction at a higher reaction temperature, tungsten in WSi.sub.2 reacts with oxygen and thus forms WO.sub.X at the surface of WSi.sub.2. After the DCS gas enters the reaction chamber and reacts with N.sub.2 O, SiO.sub.2 film is produced and deposited on top of the WO.sub.X layer and thus forming a fibrous, sponge-like appearance. It is hypothesized that during the oxide deposition process, WSi.sub.2 must provide enough Si to react with the oxidants. In amorphous WSi.sub.2 (formed by a silane base reaction), silicon can easily diffuse to the surface of WSi.sub.2 and react with the oxidant in a manner much easier than silicon in a crystalline structure can. A SiO.sub.2 film can thus be formed on the surface of WSi.sub.2 to provide a uniform layer on the surface and furthermore, to act as a diffusion barrier for the oxidants.
It is therefore an object of the present invention to provide a method of forming an oxide dielectric layer on a refractory metal silicide gate that does not have the drawbacks and shortcomings of conventional oxidation methods.
It is another object of the present invention to provide a method of forming an oxide dielectric layer on a refractory metal silicide gate by providing a diffusion barrier on the gate to prevent any reaction with the oxidants.
It is a further object of the present invention to provide a method of forming an oxide dielectric layer on a refractory metal silicide gate by providing a diffusion barrier on the gate such that the refractory metal suicide does not react with the oxidants.
It is still another object of the present invention to provide a method of forming an oxide dielectric layer on a refractory metal silicide gate by depositing a thin layer of amorphous silicon on top of the gate prior to the oxidation process.
It is yet another object of the present invention to provide a method of forming oxide dielectric layer on a refractory metal silicide gate by first depositing a layer of amorphous silicon not less than 10 .ANG. thickness on the refractory metal silicide gate before the oxidation process.
It is another further object of the present invention to provide a polycide gate structure formed of a refractory metal silicide and a polysilicon which is not affected by a subsequent oxide dielectric layer formation process.